Bicycle Frame Size Calculator

How to use it

1. Measure your inseam: stand barefoot with your back to a wall, pull a book firmly up between your legs, and measure from the floor to the top of the book.
2. Pick the bike type – road, mountain, hybrid/trekking or city – and the unit (cm or inches).
3. Enter the inseam. The frame size appears in cm and inches, with the centre-to-centre figure for road bikes.
4. Add your height to see the matching row of the size chart; if you cannot measure the inseam, the height alone gives an estimate with a warning.
5. Compare the result with the geometry chart of the model you want and test-ride the nearest sizes.

Example

Formulas, chart and limits

road (centre-to-top)  = inseam (cm) × 0.70      road (centre-to-centre) = inseam × 0.65
mountain              = inseam (cm) × 0.59      (÷ 2.54 for inches)
hybrid / trekking     = inseam (cm) × 0.685
city / comfort        = inseam (cm) × 0.68
inseam ≈ height × 0.46 when not measured

The multipliers are the long-standing fitting rules of thumb used by bike shops and the common sizing guides; they scale the seat-tube length to leg length so the saddle can sit at the right height with a sensible seat-post extension. Modern sloping-top-tube frames are often labelled S/M/L rather than in cm, and the same label can differ by 2–3 cm between brands, so treat the number as a guide to which label to try. Standover clearance (2–5 cm on a road bike, more off-road), reach and stack matter as much as seat-tube length. The height chart is the typical retail chart; it assumes average proportions. Children’s bikes are sized by wheel size, not frame size, and are not covered.

Sources: Bicycling – How to measure your inseam and pick a frame size, REI Expert Advice – Bike fit basics, Sheldon Brown – Frame sizing.

A calculator cannot replace a fitting session; if you are between sizes or have back, knee or neck problems, ask a bike fitter.

Questions

Inseam or height – which should I trust?

Inseam. Two people of the same height can differ by 5 cm or more in leg length, which moves the recommended frame by a full size. Use height only for a first look.

Why is the mountain bike size so much smaller than the road size?

Mountain frames have a low top tube for standover clearance and a long seat post; their seat tubes are therefore shorter for the same rider, and the industry quotes them in inches (17, 19, 21…) or as S/M/L.

What if I am between two sizes?

Ride both. The smaller frame is more agile and easier to adjust upwards with a longer stem and seat post; the larger one is more stable and gives a more stretched position. Reach is the deciding factor more often than seat-tube length.